A polyacetal resin is decomposed or resolved during a processing step such as extruding or molding, as a result deposits on a mold (mold deposit) are generated, and the polyacetal resin tends to be deteriorated in the moldability, the mechanical property and others. Formaldehyde generated by decomposition or resolution of a polyacetal resin is chemically active and adversely affects the heat resistance of resin. Moreover, when the resin is used as electrical or electronic parts, formaldehyde generated from the resin causes corrosion of metal contacts or their discoloration due to organic deposits, resulting in contact failure. Furthermore, formaldehyde as such contaminates the working environment for parts assembling and the ecology in the environment for using end products. The polyacetal resin is therefore required to have a high heat stability, and inhibit generation of formaldehyde during the processing step or from a shaped article of the resin.
In order to stabilize a polyacetal resin, therefore, an antioxidant or other stabilizer (e.g., a nitrogen-containing compound, and an alkali metal or alkaline earth metal compound) has been used. In particular, it is known that an aminotriazine compound such as melamine, guanamine or acetoguanamine is effective as the nitrogen-containing compound. However, since such an aminotriazine compound has a low molecular weight and also has a low compatibility with a polyacetal resin, adhesion to a mold (mold deposit) or bleeding from a shaped article (blooming) is generated along with mold processing. As a result, the aminotriazine compound brings about deterioration of formability (or moldability), defective external appearance of the shaped article, and stain due to the bled matter.
To improve such defects, a method of modifying a resin with a triazine compound increased a molecular weight thereof (e.g., a melamine-formaldehyde resin), or a salt of a triazine compound with an acidic compound (e.g., melamine cyanurate, and guanamine cyanurate), or others is being attempted. The stabilization action of the polyacetal in these modified products, however, drastically decreases compared with that in non-modified products. In addition, due to insolubility and infusibility, these products usually slightly cause the bleeding from a shaped article thereof and meanwhile deteriorate an appearance property of the shaped article due to irregularity in the surface thereof.
It is therefore an object of the present invention to provide a polyacetal resin composition which is excellent in inhibition of stabilizer bleeding, shaping processability (or moldability) and an appearance property in a surface of a shaped article formed from the composition and in which generation of formaldehyde is restrained, and a process for producing the same.
It is another object of the present invention to provide a polyacetal resin-shaped article which is excellent in an appearance property of the surface and in which generation of formaldehyde is restrained.